Watford did not disclose the cost but French media reported it was in the region of 32 million pounds, eclipsing the 18.5 million pounds the Hornets paid for forward Andre Gray in 2017.
Inter announced the deal with a video of the 26-year-old in an Inter shirt saying: "Inter: it´s not for everyone, that´s why I´m here".
The 81-cap Belgium global spent two seasons in Manchester, scoring 28 goals in 66 appearances for the Red Devils.
With Lukaku now set to finally join long-term suitors Inter, former United right-back Neville says the club will actually benefit from getting the Belgium international's alleged lack of professionalism out of the dressing room.
Lukaku joined the Serie A club on Thursday, the Premier League's transfer deadline day, but there was no new arrival to take his place at Old Trafford before the window shut.
United have included a sell-on clause as part of the deal that was completed yesterday after Lukaku arrived in Milan on Wednesday night to complete paperwork and a medical.
Liverpool to sign Adrian to replace departed Simon Mignolet
That same evening, Club Brugge announced they had signed "one of the best goalkeepers in the Premier League". Mignolet travelled straight from Wembley following the Community Shield fixture against Manchester City.
However, his departure elicited only sighs of relief at Old Trafford despite United not signing a replacement.
United have made a flurry of signings this summer and a lot of them are expected to start at Old Trafford this Sunday vs the Blues.
At 32, Mandzukic would have been a decent short-term option, but one that would best work if Man United actually had a designated out-and-out striker.
"I think, anything that happens on the last day - unless it has been ongoing - smells of panic".
Lukaku has signed a five year deal. "But I think he'll be available early next week". You can play badly, miss shots on goal, hit a bad cross or give goals away as they're mistakes in football but you can't be overweight.
Lukaku worked out at former club Anderlecht's facilities while the furor around him persisted.